There has been a great progress in the fabrication of integrated circuits in these years. High integration density makes the use of multilevel interconnections become a must. Thus, two or more layers of interconnections between circuit elements are now used. To provide vertical electrical connections between two conducting layers, contacts (or vias; by the term "contact" it is meant to cover both contact and via) are necessary. In order to achieve a better electrical connection between conducting structures or elements, the process of contact hole is quite often used to obtain aforementioned purpose. If the contact hole can not provides an effective conductive path, then some of the functions of devices will fail to perform their function. Therefore, contact hole process is one of the key ways to make sure that a layer will electrical connect to others.
Typically, the array of contact holes is regular and most of the shapes of the contact holes are squares or rectangulars. In fact, it's hard to control the CD (critical dimension) uniformity of a contact array or CD difference between dense and isolate holes. The attenuated phase shift mask becomes a mainstream as the design rule below 0.25 micron meter on deep ultra violate light (DUV) lithography process. However, at present, the exposed critical dimensions (CD) of the contact holes are not uniform in a contact hole array due to the optical proximity effect. Namely, the CD of an edge area in the array is quite different from one in center area. In addition, in areas having lager contact hole density, the exposed dimensions of contact holes are different from ones in areas having smaller contact hole density, although the original dimensions on a photomask in these two areas are the same. This effect will be more significant while the attenuated phase shifting mask (APSM) is employed, and even more when DUV lithography process is used. It sacrifices lots of process window that includes the exposure latitude (EL) and the depth of focus (DOF). Although the APSM can improve the DOF of contact hole, however, the CD difference due to the proximity effect contribution worsens than conventional mask.
A conventional way to add sub-resolution dummy holes has been proposed to minimize the CD difference between edge and center areas in a contact hole array. As shown in FIG. 1, sub-resolution lines are added around the contact holes to minimize the optical proximity effect. The method can be performed by software of optical proximity correction (OPC). Unfortunately, it has lots of limitation by using the automatic correction of software application. For example, the conversion time of the method is long, and the method is not suitable for a contact hole with a polygon shape or the like. Further, the sub-resolution method can only be applied to where the shape of the contact hole is rectangular and the configuration of contact hole array is regular. It can not be used while the shape of the contact hole is not 90-degree angled, nor while the arrangement of contact hole is irregular. There is a risk on data conversion between hierarchy and flatten structure.